In a nuclear fusion reaction, hydrogen isotope plasmas typically react in a hollow, toroidal vessel under extraordinarily high temperatures to produce helium, neutrons, and energy. Since the reactants have ionic charges, they are confined initially by a strong magnetic field within the reactor. The products of the reaction, however, are not charged, often escaping the magnetic confinement of the reactor, and striking the vessel walls (first wall or plasma facing components).
The unique properties of beryllium make it particularly desirable for application to these reactors. The properties include an extraordinarily high affinity for the gaseous contaminants of fusion plasma, a tendency to decrease disruption event frequency, a low atomic number, high thermal conductivity, and a comparatively low tritium retention. Beryllium has been found useful, for instance, as armor for the first (or entire) wall of reactors. With reactors having a modest power output, beryllium may also serve as a diverter, such as large monolithic tiles which absorb the heat generated by the fusion reaction.
State of the art reactors presently under development are expected to impose steady state heat loads on the beryllium between 0.5 and 5 MW/m.sup.2, and transient heat loads of 25 MW/m.sup.2 or higher. Unless the beryllium is cooled, these transient heat loads bring it dangerously close to its melting point in a relatively short time. Based on engineering and safety considerations, water cooled copper alloy substrates have been used to cool the beryllium. It has been found, however, that sufficient heat transfer efficiencies can only be achieved if there is a uniform bond between the beryllium and the copper alloy. Even upon water cooling the alloy, the bond line temperature has been known to reach 200.degree. to 400.degree. C. Hence, in addition to thermal criteria, a beryllium to copper alloy bond for use in fusion reactors is slated to meet a series of other relatively stringent requirements.
First, the bond should exhibit "good" bond strength (&gt;about 30 MPa) in the temperature range of 200.degree. and 400.degree. C.
Second, constituent materials of the bond should permit heat to be transferred between the beryllium and the copper alloy without becoming dangerously radioactive. Neither radioactive daughter elements nor low melting point daughter elements produced by the bonding materials are considered acceptable.
Third, large surface areas on the order of 2000 m.sup.2 should be covered by the bonded beryllium to copper alloy material. Hence, the bonding technique must be capable of being scaled readily to a selected commercial operation.
Finally, bond defect levels desired are lower than about 1%, the defect level of brazing operations presently used in Tore Supra fusion reactors.
Attempts to accommodate these requirements have included the development of a 90% In-10% Ag alloy for use in beryllium soldering, zinc, aluminum based alloys, and silver based alloys for brazing beryllium to itself. Diffusion bonding technology has also been considered, including homogeneous silver alloy interlayers and the use of homogeneous silver alloy sputtered coatings as intermediate bonding layers.
Conventional methods of bonding beryllium to copper alloys include the application of various intermediate layer materials traditionally used for brazing beryllium to itself and to other metals. However, thermal stresses have resulted from a mismatch between the coefficient of thermal expansion of beryllium and that of copper alloys. This has implications relatively important to the selection of a bonding temperature.
A uniform bond between beryllium and a copper alloy, with the requisite mechanical properties at elevated temperatures, has been attained by a silver based, vacuum induction brazing process. Although relatively good shear strengths of 220 MPa at room temperature and 150 MPa at 400.degree. C. were reported, the process uses a silver alloy as the intermediate bonding layer. Silver, it has been found, exhibits undesirable behavior upon neutron irradiation, and is considered generally unacceptable for advanced nuclear fusion applications.
Attempts have been made to duplicate the results of induction brazing, for example, by hot isostatic pressing solid beryllium to a solid copper alloy using a silver based interlayer. The resulting bond strengths, however, have been on the order of 30 MPa which are considered inferior.